The invention pertains to a method for making a flexible fibrous continuous tape from multifilament yarn, and to tapes thus made. The invention further relates to optical fiber cables comprising said tape.
Tapes reinforced with fibers (also called yarns) are known. For instance, in JP 10-130996 a carrier tape has been described: This carrier tape is produced by using a flat fiber structure having a width nearly equal to the width of the tape as a reinforcing material and a resin as a matrix. The fiber structure is a three-dimensional braid of a yarn.
In GB 1433128 a warp-knitted tape, for reinforcing matrix materials, has been described. The tape may comprise two high modulus carbon filament tows.
In WO 03/091006 a method is described of producing a novel material comprising natural fiber bundles that are pre-impregnated with organic resin and taking the form of a yarn or ribbon.
In U.S. Pat. No. 4,626,306 a method for making flexible fibrous strips has been described containing unidirectional fibers embedded in a thermoplastic resin. According to this method a lap of parallel fibers is impregnated with resin particles, by separating out fibers from a roving of fibers. A roving (or sliver) is a long and narrow bundle of fiber with a twist to hold the fiber together. The fibers are spread from the roving, but in this method only the fibers, not the individual filaments of the fiber, are spread. This method thus only provides a method that leads to surface treatment of each fiber with resin, whereas the individual filaments in the core of the fiber are not impregnated.
Similarly, in EP 316922 a method has been described wherein the fibers of a roving of fibers are unbound from the roving and are spread, followed by impregnation of the spread fibers with a resin emulsion. The individual filaments of each fiber are not spread, and again only the surface filaments of the fiber are impregnated with the resinous material.
U.S. Pat. No. 3,873,389 relates to graphite sheets which are not impregnated in-line, but first positioned in side-by-side coplanar relation, and then impregnated. The present invention does not relate to graphite and not to sheets that are not made in-line.
In the manufacture of reinforced composites having reinforcing filaments within a matrix, a filament consisting of a staple yarn spun from discontinuous fibers with low twist was disclosed in U.S. Pat. No. 5,503,928. An apparatus for continuously coating the fiber filaments wherein each filament of a fiber bundle is coated continuously in a narrow flow path of a zigzag shaped tunnel has been described in U.S. Pat. No. 6,270,851.
These known tapes thus contain fibers for reinforcing the resin matrix. Such tapes have the disadvantage that there is a relatively weak interaction between fiber and matrix. The fibers are embedded in the matrix, but only the inner filaments have an acceptable contact with the matrix. If all filaments are continuously coated, such as in U.S. Pat. No. 6,270,851, the tape contains considerable amount resin, usually 60 wt % or more, and relatively minor quantities of fiber. The resin gives a significant weight contribution to the composite (typically less than 40 wt % fiber) and the favorable properties of high-performance fibers such as aramid become less dominant as compared on a weight basis with the present invention (for example in terms of modulus, elongation at break, breaking strength). The high amounts of resin also cause an extra cost contribution in case high-performance fibers are used as well. Furthermore, the high amounts of resin decrease tape flexibility thereby causing difficulties in e.g. optical fiber cable production, since decreased bending properties may cause problems during e.g. connector positioning.
Therefore, there is an increasing need for fibers in the form of flexible continuous tapes which cannot be obtained by the known methods. In EP 837162 and US 2006/0137156 an attempt for improvement was made by spreading the filament of a multifilament yarn. Thus the filaments were spread by air blowing to better allow the resin to penetrate between the filaments. This reference however does not relate to tapes but to fiber reinforced plastics.
Similarly, in EP 569928 a method for spreading filaments and impregnation with a thermoplastic resin was described. Again this process is not intended to make tapes but to reinforce thermoplastic matrix.